Technical Tip: FortiGate WCCP mode

FortiGate as WCCP router: Intercepts HTTP and HTTPS sessions and forwards them to a web caching engine, caches web pages, and returns cached content to the web browser.

FortiGate as WCCP client: Accepts and forwards WCCP sessions and uses firewall policies to apply NAT, UTM, and more security features. Note that FortiGates may only operate as clients while in NAT mode (not in Transparent mode).

Note:

• For a FortiGate acting as a WCCP Client, when the WCCP setting is enabled on a firewall policy to offload WCCP sessions to an external device, use the appropriate source and destination in that policy.
• If the source and destination fields are 'all' on the firewall policy, the sessions will be offloaded to a WCP-compatible device, and if that device is not meant to accept all the traffic, the packets are simply dropped.

Related articles:

system wccp 

Sample configuration of FortiGate WCCP server and client (traffic redirector and transparent proxy), with verification and troubleshooting procedures.

WCCP configuration between 2 FortiGates using GRE tunnel (Router/Server and Client):
Technical Tip: Sample configuration of FortiGate WCCP server and client (traffic redirector and transparent proxy), with verification and troubleshooting procedures

WCCP configuration between FortiGates (Router/Sever) and squid proxy using GRE tunneling:
Technical Note: Configuring WCCP interception of HTTP traffic to a squid proxy using GRE tunneling